feat: add first valid response timing and related metrics

- Introduced `first_valid_response_ms` and `ttfr_ms` to track the timing of the first valid response in LLM calls.
- Updated relevant interfaces and components to display and utilize the new metrics, including CallDetails, CallTable, and LatencyChart.
- Enhanced the database schema to accommodate the new timing fields.
- Implemented logic in the service layer to record the first valid response during model interactions.

server/migrations/0005_add_first_valid_response_timing.sql

@@ -0,0 +1,2 @@
+ALTER TABLE llm_calls ADD COLUMN first_valid_response_at_ms INTEGER;
+ALTER TABLE llm_calls ADD COLUMN ttfr_ms INTEGER;